A bicone is generally an antenna having two conical conductors where the conical elements share a common axis and a common vertex. The conical conductors extend in opposite directions. That is, the two flat portions of the cones face outward from one another. The flat portion of the cone can also be thought of as the base of the cone or the opening of the cone. The flat portion, or opening, of a cone is at the opposite end of the cone from the vertex or point of the cone. Bicone antennas are also called biconical antennas. Generally, a bicone antenna is fed from the common vertex. That is, the driving signal is applied to the antenna by a feed line connected at the antenna's central vertex area.
Positioning two cones so that the points (or vertices) of the two cones meet and the openings (or bases) of the two cones extend outward (opposite one another) results in a bowtie-like appearance.
Generally, bicone antennas support a wide bandwidth, but the low end of the operating frequency range is limited by the aperture size of the antenna, which is the overall length of the antenna along the bicone surface. The relationship between aperture size and frequency operation is generally inverse. That is, operation at a lower frequency requires a larger bicone antenna. More specifically, a traditional bicone antenna requires an aperture size of about one half of the longest operating wavelength. The longest wavelength is related to the lowest operating frequency by the wave velocity relationship, “speed of light=wavelength×frequency” where the speed of light is approximately 300,000,000 meters per second.
Lower frequency operation suggests a bicone antenna with an increased electrical length. Increased length often means increased width. This increased electrical length maintains a low VSWR (voltage standing wave ratio) at the lower operating frequencies. This translates into improved matching and thus signal coupling into the antenna. In contrast, higher frequency operation suggests a smaller electrical length. While a bicone antenna with increased electrical length will operate at these higher frequencies, the resulting radiation pattern is generally less effective as more energy is directed upward than out along the horizon.
Accordingly, there is a need in the art for an omni-directional bicone antenna having both a long electrical length for low frequency operation and a reduced electrical length during high frequency operation.